The ones I have look just like that and haven't grown much at all. I do have some very fast growing sponge, but it doesn't like lots of light and stays under the rockwork for the most part. Helps glue all the rock together. In fact, none of my other sponges grow quickly at all. I have a bright yellow one that's been in my tank for years now, and a shiny black sponge grows beside it. Colors contrast quite well.
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Brad
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